1. Field of the Invention
The present invention relates to a method of manufacturing a magnetic particle, a magnetic particle and a magnetic recording medium containing this magnetic particle in a magnetic layer.
2. Description of the Related Art
Reducing the particle size of a magnetic particle contained in a magnetic layer is important for increasing the magnetic recording density. For example, in magnetic recording media widely used as video tapes, computer tapes, disks, etc., noise decreases when the particle size is reduced in a case where the mass of a hard magnetic material is the same.
A CuAu type or Cu3Au type hard magnetic ordered alloy is attracting attention as a material for a magnetic particle which is promising for improving the magnetic recording density (described, for example, in Japanese Patent Application Publication No. 2003-6830 and Japanese Patent Application Publication No. 2001-256631). Because this hard magnetic ordered alloy has large crystal magnetic anisotropy because of strains generated during ordering and it is known that this alloy shows hard magnetism even when the particle size of a magnetic particle is reduced.
Although a magnetic particle showing hard magnetism is prepared by a liquid phase process, a gaseous phase process, etc., a magnetic particle immediately after the preparation of a practical liquid phase process excellent in mass producibility, in particular, has a structure of an unordered face-centered cubic crystal. A face-centered cubic crystal usually shows soft magnetism or paramagnetism and is unsuitable for a magnetic recording medium. In a magnetic recording medium, therefore, it is necessary to obtain a hard magnetic ordered alloy having a coercive force of not less than 95.5 kA/m (1200 Oe). In order to obtain such a hard magnetic ordered alloy, it is necessary to cause an unordered phase to transform to an ordered phase. For this reason, the manufacturing of a magnetic particle comprises the alloy particle preparation step of preparing an alloy particle capable of forming a CuAu type or Cu3Au type hard magnetic ordered alloy phase and the magnetic particle formation step of forming a CuAu type or Cu3Au type magnetic particle from the alloy particle formed in this preparation step, and in the magnetic particle formation step, usually annealing treatment (heat treatment) is performed.
However, whether the performance of a magnetic particle used in a magnetic recording medium is good is not determined by the magnetic particle formation step alone, and the key point is how to prepare, in the alloy particle preparation step, an alloy particle which has a micro size, is excellent in monodispersibility and provides an excellent transformation facilitativeness which facilitates the transformation of an unordered phase to an ordered phase. Usually, the preparation of an alloy particle is performed by mixing a plurality of kinds of solutions for alloy particle preparation by the above-described liquid phase process, and a device in which a stirring vane is provided in a tank as shown in FIG. 13 has been proposed as a mixing and reaction device (Japanese Patent Application Publication No. 5-173267).
In this mixing and reaction device 1, in order to stir solutions filled in the interior of a tank 2, a stirring vane 5 is rotated and driven via transmission device 4 which transmits the rotational driving force of a motor 3 in a noncontact manner by use of magnetic force. On the outer circumferential surface of the tank 1 is provided temperature control device 6, which heats or cools the solutions in order to control the temperature control of the solutions filled in the interior. A sealing lid 7 of this tank 2 is provided with an introduction pipe 8 which introduces the solutions and the bottom of the tank 2 is provided with a discharge pipe 9 which discharges mixed reaction solutions which are mixed and caused to react within the tank 2.
However, in the preparation of an alloy particle by use of the conventional mixing and reaction device, a dead space is present within the tank 2, making the stirring of the solution nonuniform, and particle growth occurs due to the partial circulation of a formed alloy particle within the tank 2. Therefore, the conventional preparation of an alloy particle has the drawback that it is impossible to produce an alloy particle which has a micro size, is excellent in monodispersibility and provides an excellent transformation facilitativeness.
Furthermore, magnetic recording media which have recently been widely used as video tapes, computer tapes, disks, etc., are required to meet the requirement for a further noise reduction.